The present invention relates to improvements made to safety devices for gas burners, particularly in a cooking appliance, and particularly those suited to be mounted on a built-in hob equipped with gas burners. The invention is aimed more specifically at improving a safety device for a gas burner according to the preamble of claim 1.
The gases used as fuel to produce energy, and which when mixed with air under certain conditions produce explosive mixtures, are dangerous and require the use of safety devices. In particular, valves for cooking appliances employing gas burners, in addition to having a flow-regulating control valve for the gas, are also equipped with such devices. These in particular allow the gas supply to be cut off as soon as the gas is not being burnt in a burner.
These safety devices generally employ a thermocouple, arranged in the burner, to monitor its correct operation and a safety shutter which, in the event of failure, allows any circuit supplying gas to the burner to be shut off. When the burner is operating normally, the heat from the flame causes a difference in electric potential in the thermocouple. This potential difference is used to activate an electromagnet which keeps the safety shutter against a compressed spring; when the shutter is in the open position, that is to say it does not prevent the gas from flowing through the supply circuit. The safety device is in the set position, ready to perform its function. Thus, in the event of failure, for example if the gas is no longer being burnt in the burner, and therefore near the thermocouple, the potential difference disappears, the electromagnet is no longer activated and the safety shutter is free to move under the impulse of the spring, and this drives it into the closed position to shut off the gas supply. The same thing happens when the gas flow-regulating control valve is closed, that is to say that it prevents gas from being supplied to the burner, because then no flame can heat up the thermocouple.
The simple manipulating of the valve by an operator, that is to say the opening of the supply-regulating control valve to a greater or lesser extent, therefore has no influence on the supply to the burner as long as the device is not set, that is to say as long as the safety shutter is in the closed position. There are therefore mechanical setting devices that allow the shutter to be kept open, by deliberate action on the part of the operator, at least until the burner has been lit and enough of a potential difference has been created to activate the electromagnet.
A rotary knob is generally situated above a casing of the hob. Through it being turned in a horizontal plane, it allows the extent to which the valve is opened to be adjusted, that is to say it allows the desired gas flow rate to be set. This knob also, through a vertical upward movement, allows the safety device to be set. It is a vertical control rod, fixed under the knob or operated indirectly by the knob, which transmits the setting instruction, that is to say the upward movement, to the setting device.
Safety and setting devices are generally included in the body of the valve and arranged in the supply circuit, this making it possible to make savings in terms of compactness and to limit the number of gas seals required between excessively numerous components of the supply.
A casing of a built-in hob must be fairly slim, generally thirty millimeters thick, in order to comply with the standards of kitchen furniture manufacturers. Most of the components of the valve are therefore arranged horizontally inside the casing. This is the case of the shutter which moves horizontally under the opposing influences of the setting device and of the spring. The setting device has therefore to allow the vertical control movement of the control rod to be converted into a horizontal movement for opening the shutter.
A first type of safety valve comprises a setting device one part of which transmits the vertical translational movement to a cranked lever, articulated for rotation, which converts the vertical movement into a horizontal movement. Such a device entails arranging several parts and their articulations with precision within the body of the valve. These arranging operations cannot be done by automatic means but must be performed manually. In order to use these parts, the valve body is manufactured in two bits, by casting or forging, one to accommodate the parts and the other to act as a lid for the first. A seal is also arranged between the two bits. A valve such as this is expensive and its setting device is delicate in proportion to its complexity.
A second type of valve comprises a setting device which uses two ramps each mounted for translation, sliding one along the other, and the slopes of which are designed to convert the vertical movement of one ramp into a horizontal movement of the other. Such a device also has disadvantages. The precision of its production and of its mounting has to be very high given the small travel of the ramps. Further, as one ramp does not bear on the other axially, it generates radial forces and wear in bearings for guiding them and the rubbing-together of the ramps causes them both to wear. This wear may cause valve malfunction.
Document GB 2 261 495 describes a safety device of the type mentioned above, in which the setting lever is in the form of a solid and rigid cylindrical bar, of one piece with its two radial arms which are also solid and rigid. Such an arrangement allows no offset between the respective travels of the actuating rod and of the safety valve shutter rod. This demands both high precision in the manufacture of the setting lever and of its arms, which makes it an expensive part, and in the adjustment of the parts during assembly, which increases the cost of manufacture.
A European standard dictates that a safety valve be capable of performing at least 40,000 operations without failing. All the parts have therefore to have good resistance to the friction to which they are subjected. Significant friction leads therefore to high costs of manufacture of these parts.